Silica is a very common stuff in the ground. Chemists write it SiO2: silicon joined to oxygen. You already know it by other names. Beach sand is mostly silica. Quartz is silica grown into a hard, glassy crystal. Flint is silica too — the dark stone that can strike a spark. Glassmakers melt silica (with a few other things) to make glass, so the word turns up in science class and in factories, not much in the kitchen. Though actually, the tidy picture — “silica is sand” — is a little too tidy. Sand is mostly silica, but it can carry other grains as well; and “silica gel,” those little packets in a new bag that say do not eat, is still silica, only made into a dry, thirsty form that drinks water from the air. That gel is not the soft rubber you bake in. That is silicone, a different word.